This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to incorporation of a method for compensating both cross-track repeatable written-in repeatable run-out error and cross-track non-repeatable written-in repeatable run-out error of a disc drive.
Disc drives are used for data storage in modern electronic products ranging from digital cameras to computers and network systems. Typically, disc drive includes a mechanical portion, or head disc assembly (HDA), and electronics in the form of a printed circuit board assembly (PCB), mounted to an outer surface of the HDA. The PCB controls HDA functions and provides a communication interface between the disc drive and a host being serviced by the disc drive.
Typically, a HDA includes a magnetic disc surface affixed to a spindle motor assembly for rotation at a constant speed and an actuator assembly positionably controlled by a closed loop servo system. The actuator assembly supports a read/write head that traverse generally concentric magnetic tracks radially spaced across the disc surfaces. Disc drives using magneto resistive heads typically use an inductive element to write data to the tracks in the form of magnetic flux transitions and a magneto resistive element to read data, such as servo data, from the track during drive operations. Servo data are typically written to the track during the manufacturing process by a servo track writer and are used by the closed loop servo system for controlling read/write head position during drive operations.
Continued demand for disc drives with ever-increasing levels of data storage capacity, faster data throughput and decreasing price per megabyte have led disc drive manufacturers to seek ways to increase the storage capacity and improve overall operating efficiencies of the disc drive. Present generation disc drives typically achieve aerial bit densities of multiple gigabits per square centimeter, Gbits/cm2. Increasing aerial bit densities can be achieved by increasing the number of bits stored along each track or bits per inch (BPI), generally requiring improvements in the read/write channel electronics, and/or by increasing the number of tracks per unit width or tracks per inch (TPI), generally requiring improvements in servo control systems.
An approach taken by disc drive manufacturers to improve servo control systems has been through the introduction of methods for compensating repeatable run out error (RRO). RRO error is introduced into a servo burst of the disc drive during a servo write process. RRO error negatively impacts the alignment of the read/write head relative to track center of the data track by causing the data track formed during the servo write process to be an irregular, generally circular shape rather than a desired substantially perfect circle. Through incorporation of appropriate correction factors, the original irregular, generally circular shaped data track becomes a substantially perfect circle.
One such construction of RRO error compensation recently proposed in the art is exemplified by U.S. Pat. No. 6,069,764 issued to Morris et al. The Morris solution incorporates a transformation of a sequence of time domain repeatable run-out values into a sequence of frequency-domain repeatable run-out values, dividing the frequency-domain repeatable run-out values by measured transfer functions of the servo system at selected frequencies, then inverse transforms the frequency-domain sequences of compensation values to produce a sequence of time domain compensation values and injects the time domain sequence of compensation values into the servo loop to compensate for the RRO error. The basic method used to compensate RRO error is referred to as Zero Acceleration Path (ZAP). ZAP uses a position error signal (PES) generated from a servo burst written on the data track during the servo write process to determine the real RRO error and generate correction factors. However, the existing method to determine the RRO error is insufficiently accurate to meet the demands of ever-increasing TPI requirements with shrinking total measurable run-out (TMR) budgets, since current methods cannot separate cross-track repeatable written-in repeatable run-out error (CTR-WIRRO) and cross-track non-repeatable written-in repeatable run-out error (CTNR-WIRRO) components of the total RRO error. CTR-WIRRO derives from physical or mechanical events such as disc slip and vibration emanating from spindle imbalance. Large CTR-WIRRO error is beyond capabilities of ZAP and reduces the efficiency for written-in RRO error compensation. Also, CTNR-WIRRO error is the primary component of the total RRO error that contributes directly to write-to-read and write-to-write track mis-registration. As track densities continue to increase and design budgets for (TMR) decrease, challenges remain and a need persists for improved techniques of dealing with a wider spectrum of repeatable error components contributing to a total position error signal to assure a reduction in write-to-write and write-to-read mis-registration to improve data integrity of information recorded on adjacent tracks of the disc of the disc drive.
The present invention provides a method and apparatus for isolating and correcting cross track repeatable and cross track non-repeatable written-in repeatable run-out error components of a total written-in repeatable runout error of a disc drive through a determination of the total written-in repeatable run-out error written-in to a servo sector of a data track of a rotatable disc surface of the disc drive, isolating the cross track repeatable written-in repeatable run-out error component of the total written-in repeatable run-out error, removing the cross track repeatable error component from the total written repeatable run-out error to provide a cross track non-repeatable written-in repeatable run-out error component of the total written-in repeatable run-out error and providing both the repeatable and non-repeatable error value components to a processor.
The processor provides providing a cross track repeatable compensation signal, for application to a servo control circuit of a servo loop of the disc drive by a repeatable run-out error compensation circuit to compensate the cross track repeatable written-in repeatable run-out error component of the total written-in repeatable run-out error.
The processor also provides a cross track non-repeatable compensation signal, for application to the servo control circuit of the servo loop of the disc drive by a non-repeatable run-out error compensation circuit to compensate the cross track non-repeatable written-in repeatable run-out error component of the total written-in repeatable run-out error.
These and various other features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.